Under cold start conditions, a vehicle engine and powertrain may have cooled to ambient conditions. Consequently, each component of the engine and powertrain has to be warmed-up to a desired operating temperature. The time taken to overcome the lower powertrain temperature and reach an optimum operating temperature may be significantly large. While the engine warms up, there may be high friction within the engine due to a higher viscosity of engine fluids such as engine oil when they are relatively cold, and further, heat may be lost to engine coolant thereby reducing a thermal efficiency of the engine. Overall, these effects may lead to a lower fuel economy, an increase in engine wear, as well as an increase in exhaust emissions. Thus, accelerating an engine warm-up can provide various benefits.
Various approaches have been developed to expedite engine heating during an engine cold-start. For example, engine intake air may be heated via a dedicated heater. In still other approaches, fuel injection and/or ignition timing adjustments are used to expedite heating during start-up. The inventors herein have recognized that heat transfer to the engine internal components can be further improved by heating the fresh air entering the crankcase. In particular, this may allow for higher rates of heat transfer at low ambient temperatures. As such, various approaches have also been developed for heating crankcase gases. For example, intake air may be heated via a heating assembly coupled to the air induction system, such as shown in U.S. Pat. No. 2,797,674. As another example, as shown in US 20050016474, intake air may be delivered to a crankcase after passing through a heating box, the heating box warmed via radiant heat from engine cylinders and exhaust gas.
However, an issue with such approaches is the need for a dedicated heater, which can add component complexity and costs. In addition, the operation of the heater may lead to a fuel economy penalty. Further still, heating of the intake manifold air can worsen the engine's knock limit, and decrease the engines maximum power output. In systems relying on heat exchange via a coolant, the maximum coolant temperature may limit the amount of heat that can be transferred. In addition, the relatively slow warm-up of a coolant may limit the portion of a trip time that can be utilized to heat intake air or crankcase gases. Thus, heating the engine intake air may be substantially different from heating the air intentionally entering the crankcase. It is also different from heating the fittings through which the crankcase air passes as it leaves the crankcase (which is done for freeze avoidance).
In one example, some of the above issues may be at least partly addressed by a method for an engine, comprising drawing fresh air through an interstitial space of a double wall exhaust system to heat the air, and then directing the heated air to a crankcase. In this way, engine heating may be expedited.
As an example, an engine may include a double walled exhaust manifold that is configured as an exhaust-to-air heat exchanger. Fresh intake air (boosted or unboosted) is drawn, from upstream of an intake throttle, through an interstitial space of the double walled exhaust manifold to heat the air with exhaust heat. The heated air is then directed to a crankcase for positive crankcase ventilation (PCV). This allows heat to be transferred to engine internals. The transfer rate increases as the ambient temperature decreases, allowing for higher heat transfer at engine cold-start conditions. In other words, more heat is serendipitously transferred to a location suited for engine heating during conditions when rapid engine warming is most required. Crankcase vapors released from the crankcase are then received in the engine intake manifold, downstream of the intake throttle. A temperature of the crankcase vapors, however, may be lower than the temperature of the heated air entering the crankcase. For example, the crankcase vapors may be at the bulk temperature of the crankcase. In other words, heating the crankcase air does not lead to a corresponding increase in the temperature of crankcase vapors entering the engine. As a result, issues related to heating of the intake manifold air, such as worsening of a knock limit, or a decrease in the engines maximum power, are averted. The approach may also be used with crankcase ventilation systems that operate at largely constant flow rate to further improve blow-by gas oil separation.
In this way, the double wall exhaust manifold as described herein establishes a synergy in functionality, in that heated intake air can be drawn into the crankcase to expedite engine heat transfer precisely when engine heating is desired, without heating the crankcase gases delivered to the engine intake. By expediting engine heating during an engine cold-start, fuel economy, engine performance, and exhaust emissions improvements are achieved. As such, freezing of the crankcase effluent in the exit conduit can occur at cold ambient temperatures and can be exacerbated by cold, moist ventilation air entering the crankcase. Herein, by delivering heated air to a crankcase, the need to heat PCV fittings, to prevent icing, is reduced. As such, this leads to further improvements in fuel economy and improvements in PCV performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.